1. Field of the Invention
The present invention relates to photosensing elements and, in particular, to a process for manufacturing a capacitor coupled contactless imager that uses poly1 as the phototransistor emitter contact and poly2 to form the gates of the peripheral CMOS transistors and to partially cover the phototransistor base region as the coupling capacitor. This approach results in higher capacitive coupling and smaller pixel cell size than prior art imagers, thereby providing higher resolution and wider dynamic range.
2. Discussion of Related Art
Eric Fossum, "Active-Pixel Sensors Challenge CCDs", Laser Focus World, pp. 83-87, June 1993, discusses emerging active-pixel sensor technology that is poised to replace charge coupled device (CCD) technology in many imaging applications.
As discussed by Fossum, a CCD relies on charge shifting to read out an image. Since it is very difficult to achieve 100% charge transfer efficiency in a CCD structure, performance is sometimes degraded below acceptable levels. In contrast to CCD technology, an active-pixel sensor operates similarly to a random access memory (RAM), wherein each pixel contains its own selection and readout transistors. The signal readout then takes place over conductive wires rather than by shifting charge. Thus, active pixel sensor technology provides advantages over CCD technology such as random access, nondestructive readout and intergrability with on-chip electronics.
In U.S. Pat. No. 5,289,023, issued Feb. 22, 1994, Carver A. Mead discloses a photosensing pixel element that uses a bipolar phototransistor as both an integrating photosensor and a select device. In Mead's preferred embodiment, the phototransistor is a vertical structure having its collector disposed in a substrate of N-type silicon. The base terminal of the bipolar phototransistor, which comprises a p-doped region disposed within the collector region, is utilized as the select node for the pixel. Conventional field oxide regions are employed to isolate the base regions of adjoining phototransistors. An n-doped polysilicon line is disposed over the surface of the substrate and is insulated therefrom except in regions where it is in contact with the p-doped base regions. Where the n-doped polysilicon is in contact with the surface of the p-type base region, it forms an n+ epitaxial region which serves as the emitter of the phototransistor. The polysilicon line provides the emitter contact.
As further disclosed in the '023 patent, a plurality of the Mead phototransistors may be arranged in an array of rows and columns. The bases of all phototransistors in a row of the array are capacitively coupled together to a common row-select line, and the emitters of all phototransistors in a column are integral with a column sense line. The input of a sense amplifier is connected to the sense line of each column of integrating photosensors. The sense line is connected to the inverting input of an amplifying element of an integrating sense amplifier. A capacitor, preferably a varactor, is also connected between the inverting input and the output of the amplifying element. Exponential feedback is provided in the sense amplifier for signal compression at high light levels. The outputs of the sense amplifiers are connected to sample/hold circuits. The rows of the array are selected one at a time and the outputs of the sample/hold circuits for each row are scanned out of the array while the pixel data for the next row are sampled.
U.S. Pat. No. 5,289,023 is hereby incorporated by reference in its entirety.
In the operation of the pixel imager array disclosed in the '023 patent, the emitter current of the pixel during a read operation is typically in the .mu.A range. Therefore, the junction leakage currents set a limit on the dynamic range at low light levels and should be as small as possible. Furthermore, the base coupling capacitor needs to be large compared with the junction capacitance of the emitter, base and collector such that during the image integration period, the base remains reverse biased with respect to the emitter and collector.